FIG. 1 shows a ducted fan gas turbine engine 10 comprising, in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36. The engine has a principal axis of rotation 44.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by interconnecting shafts 38, 40, 42.
It is commonplace in state of the art gas turbine engines 10 to include variable vanes at various locations in the engine to generally help control the air flow passing through the engine core, thus improving the performance of the engine. FIG. 2 illustrates a typical Variable Stator Vane assembly 210 which is used in various engines for aerospace, industrial and marine applications.
The assembly includes four unison rings 212 coaxially arranged relative to the principal axis 44 of the engine 10 around the exterior of the intermediate compressor 18 (not shown in FIG. 2). Each unison ring 212 has a plurality of lever arms 214 each of which attach to a vane spindle (not shown) via the mounting holes 216 located in the distal end thereof. The aerofoil portions of the vanes are rotatably mounted within the airflow path of the compressor 18 such that they can rotate about the major axis of the vane which is coincidental with the rotational axis of the spindle. The unison rings 212 are arranged such that rotating them around the principal axis of the engine results in the lever arms 214 pivoting about the vane spindles, thereby rotating them and the aerofoil portions within the airflow channel of the compressor 18.
To rotate the unison rings, an input link is provided to engage with a crankshaft which drives the unison rings via link rods. As the levers rotate around the centre of rotation of the vane spindles, the lever arms fixed interface with the unison ring requires that the unison ring translates axially as well as circumferentially as it translates.
This design requires that all the interfaces between moving components have bushes fitted to accommodate the necessary movement. Additionally, to keep the unison rings circular as they accommodate the loads within the system, the rings are fitted with centralising features that contact the casings to maintain roundness. This can be achieved by using mushroom headed bolts or centralising screws which contact pads that are bonded or bolted to the engine casing.
This assembly is unnecessarily complex and heavy. Thus, the present invention seeks to provide an alternative arrangement.